The fungal species Aspergillus niger is used widely in the industrial production of enzymes, e.g. for use in the food industry. Its use is based on the secretory capacity of the microorganism. A. niger is used in large quantities and on a commercial scale for this industrial production, and is consequently a well-characterized micro-organism. To apply genetic engineering techniques to A. niger, it is necessary to transfer DNA into A. niger so that the resultant fungal transformant may express an additional useful product and secrete it in large amounts.
Numerous genes have been cloned in a variety of prokaryotic vectors, and efforts are being made to obtain high expression levels for the encoded proteins. Genes are likely to be more efficiently expressed and processed and to produce a product more nearly the same as the original protein if these genes are placed into a host that is the same as or closely related to the species from which the gene was originally extracted.
However, very high levels of expression of individual proteins can be deleterious, if not lethal, to the cell if allowed to build up to high concentrations therein, and can cause crystallization of the protein into inactive and highly insoluble inclusion bodies. Thus one should develop systems that allow efficient excretion of synthesized proteins from the cell. Also, it is best to chose as hosts organisms which have been grown commercially, so that the already well developed fermentation technology and downstream processing can be exploited in the production of novel products or the increased production of existing products. A. niger is the source organism for numerous industrially important proteins, enzymes and other products. It is also capable of excreting highly expressed proteins. The transformation of filamentous fungi such as A. niger has however proved particularly difficult.